Resistance evaluation of walnut (Juglans spp.) against Xanthomonas arboricola and the correlation between leaf structure and resistance

Abstract

AbstractWalnut blight caused by Xanthomonas arboricola pv. juglandis (Xaj) is one of the most detrimental diseases affecting walnut quality and production. A screen of 18 walnut genotypes for leaf resistance to Xaj suggested species independent variation and revealed four highly resistant (HR) cultivars (‘96’, ‘91’, ‘Chuanzao2’ and ‘Qingxiang’), while ‘Yanyuanzao’ and ‘Xiangling’ were highly susceptible (HS). Further correlation analyses were carried out on the 18 genotypes. Stomata length (SL) (r = .686, p < .05) and stomatal area (SA) (r = .596, p < .05) had significant positive correlations with disease severity index (DSI), suggesting that the length and density of stomata affected the speed and number of pathogens invading the leaves. The highly susceptible genotypes had a significantly lower thickness ratio of spongy to palisade tissue (P/S) (1.50). When compared to resistant genotypes (1.69), the P/S value had a significant negative correlation with the DSI (r = −.631, p < .05). Denser spongy tissue and more developed palisade tissue were indicative of higher resistance for the genotype. The stomata breadth (SB), stomata density (SD) and wax content (WC) each had no correlation with resistance. Maximum SW, SD and TU and minimum WC were found in HS genotypes, such as ‘Xiangling’ and ‘Yanyuanzao’. The leaf thickness (LT), the thickness of the abaxial epidermis (TL), the thickness of stratum corneum (LCT), the tissue structure tense ratio (CTR) and tissue structure loose ratio (SR) did not correlate with resistance. These findings not only provide information to facilitate our understanding of Xaj resistance but also lay the foundation for breeding Xaj‐resistant walnut in the future.